im pretty sure caps 7 and 8 are to help with noise floor, but if you use the right cap (nichicon muse es for example) it improves the sound, especially bass handling. id put them in there its maybe 2$ more in parts.

as for the resistors, 1/2 watt will work absolutely fine. they are considered overkill, but there only detriment besides cost is that they are physically larger. 1/4 watt is still on the safe side, there cheaper (not by alot, but still) and their smaller. size isnt a big concern as you are not using a production board, but if you were, you would need the 1/4 watt resistors becasue 1/2 wouldnt fit on the board.

On your previous order you had 1/4 watt 2k ohm resistors shown. Now everything is 1/2 watt. Either one would be fine, or like kchapdaily mentioned you could use 2 or the 1k in series, or 2 of the 4k parallel.

C7 and C8 are decoupling (or bypass) capacitors. They decouple the signal being developed across R5 and R11. They are pretty important and a basic element to the design of the amplifier.

They are not a basic element to the design of the amp, Pete didn't have them implemented in his original design. They were added in when dsavitsk (I believe) added in his improvements, and can be left out. I know for a fact that Beftus had at least one SSMH where he disconnected C7/C8, he noted that it reduced the overall gain but otherwise didn't change too much.

As for pin 9 on 12AU7, it is used for a center tap (instead of 12.6V, 6.3V) and can be utilized in some single-tube designs (like the Bravo amps on eBay). We don't need it here so it's not connected to anything.